1. Field of the Invention
The invention concerns a microphone with adjustable properties.
2. The Prior Art
Membrane microphones include a membrane which is placed over a back chamber. Further, these microphones are equipped with a small vent opening ventilating the back chamber to the surroundings. This is necessitated by changing pressure conditions in the atmosphere. This opening is also known as a barometric relief opening.
In many acoustical devices, such as hearing aids, there is a need to control the sensitivity of the microphone especially at low frequencies. This could, as an example work as a remedy against unwanted signals at very low frequencies, in particular wind-induced noise.
One way of decreasing the above-mentioned acoustical low frequency noise from wind noise and other sources is to use a microphone with a large size vent opening from the inside of the microphone to the surrounding air. This effectively short circuits the low frequency signals since the opening will equalize the pressure changes provided that they are sufficiently slowly varying as in the case of low frequency noise.
If the vent opening is small wind-induced noise and other low frequency noises are a problem, whereas a large vent-opening decreases the sensitivity towards wanted low frequency signals. The invention presents a solution to this dilemma.
The vent opening is very important for the properties of any microphone. It is well known by people skilled in the art that the pressure equalization due to the opening may be described by a simple 1. order high pass filter function as described in EP Patent publication EP 0 982 971 A2,
      L    ⁡          (      ω      )        =            j      ⁢              ω                  ω          l                            1      +              j        ⁢                  ω                      ω            l                              
where ωl is the corner frequency for the low frequency rolloff. The corner frequency may move to higher or lower frequency according to the size of the vent-opening as described in the above cited publication. Hence the size of the opening determines the compromise between sensitivity towards useful signals versus noise.
Situations also exist where the microphone sensitivity towards low frequencies should be changeable according to the environment in which the microphone works.
One example of such a situation is the already mentioned wind noise situation, where the microphone is to react to increase in wind noise. Another example is a microphone sensing the acoustic signals existing in the ear canal of a wearer of the hearing aid. Such a microphone may be located in the hearing aid on the side pointing into the ear. This additional microphone may be useful in connection with active countermeasures against the experience of occlusion due to the hearing aid as described in Danish patent application PA 2002 01292. Such an internal microphone works in a special environment where the individual size of the residual cavity behind the hearing aid and in front of the eardrum affects the optimum low frequency response of the microphone system. Therefore a microphone with an adjustable vent opening would form an important part of the anti-occlusion system.
A further relevant application is when using two or more microphones together in order to obtain directional patterns. In such cases it is important that all microphones have the same high pass filter function. Any deviation in filtering characteristic between the individual microphones will lead to phase problems in the directional system and the directionality will suffer at low frequencies. By means of adjustment of the corner frequencies the filtering in the two microphones can be matched and the directionality can be maintained at low frequencies. This is not possible with present day microphones.
It is the object of the invention to provide a microphone which overcomes the short-comings of a conventional microphone.